1. Field of the Invention
This invention relates to broadband communications and more particular to saving bandwidth in a two-way communication system.
2. Background of Related Art
Generally, in two-way communication systems the total system bandwidth is usually divided into two segments (i.e., frequency bands). Inbound and outbound information each have a dedicated bandwidth. Accordingly, the total bandwidth for a given direction is only half of the overall system bandwidth.
For example, in a point-to-multipoint transmission system such as shown in FIG. 1, the central office or head end (CO/HE) 10 broadcasts downstream information over a single transmission line 102 to remote non-switch splitter 19 that further distributes the information to multiple end users 30, 40, 50, 60 over transmission lines 130, 140, 150, 160, respectively. The end users (EUs) also send upstream signals over the lines 130, 140, 150, 160 to the remote splitter 19 which further forwards that information to CO/HE 10 over the single transmission line 104.
Because of the passive splitting of the splitter 19, all users can receive all the downstream information by tuning to the right channel (frequency, time slot, etc.). Therefore, no privacy can be assured. Also, because each EU uses the same physical line for bi-directional transmissions (lines 130, 140, 150, 160), the downstream and upstream traffic need to be allocated into non-overlapped bands (frequency, time slots, code, or wavelength, etc.). Since no switch function is deployed at remote splitter 19, each EU also needs to follow a certain algorithm (i.e, Frequency Division Multiple Access: FDMA; Time Division Multiple Access: TDMA; Wavelength Division Multiple Access: WDMA; or Code Division Multiple Access: CDMA) to avoid collision with other EU transmission over the shared upstream transmission line 104.
In a system deploying Frequency Division Multiplexing (FDM) for downstream transmission and FDMA for upstream transmission, the bandwidth allocation is shown in FIG. 2. Downstream information A.sub.D, B.sub.D, C.sub.D and D.sub.D for EUs 30, 40, 50 and 60 are broadcast from CO/HE 10 to remote splitter 19 over single transmission line 102. Downstream information A.sub.D is intended for EU 30, downstream information B.sub.D is intended for EU 40, downstream information C.sub.D is intended for EU 50 and downstream information D.sub.D is intended for EU 60. The remote splitter 19 further distributes all the downstream information to all the EUs. Each EU then tunes to the right frequency band to receive the information that is specifically addressed to that specific EU. For upstream transmission, each EU will transmit its signals in the frequency band that is different from other EUs and also non-overlapped with downstream frequency bands as shown in FIG. 2. For example, the downstream information is grouped into bandwidths having a lower frequency using a low-pass filter while the upstream information is grouped and transmitted in bandwidths having a higher frequency using a high-pass filter. The downstream and upstream bandwidths each generally occupy one-half of the total frequency spectrum.
Based on the same principle, in a system deploying Time Division Multiplexing (TDM) for downstream transmission and TDMA for upstream transmission, the downstream information will be broadcasted to all the EUs (TDM). Each EU will tune to the right time slot to receive its signal. Each EU will use the time slot assigned to this EU for upstream transmission (TDMA). Because single lines 130, 140, 150, 160 are used for both upstream and downstream transmission (assume using same frequency, optical wavelength, or code), Time Division Diplexing (TDD, also called Ping-Pong) may be used to avoid conflict between downstream and upstream traffic.
Two disadvantages of the above systems are the waste of bandwidth resource by allocating the downstream and upstream traffic into two different bands and the loss of privacy.